This invention relates to a resonance control and protection method and system for use in connection with a controllable power capacitor device. A controllable power capacitor device typically includes one or more power capacitor banks and a programmable control unit. It is known that resonance conditions can be created when such devices are utilized in applied power transmission and distribution circuits. Such resonance conditions may be due, e.g., to capacitor switching and/or system changes. The present invention seeks to provide a mechanism for detecting and remedying resonance conditions in power transmission and distribution systems. A further goal of the invention is to eliminate or alleviate the need for costly and complicated system studies, which are conventionally required when using power capacitor devices, and to simplify the design and application of controllable power capacitor devices.
Capacitor banks have been widely applied by utilities and customers with industrial distribution systems, both to provide voltage support and to improve the power factor of a load. Such capacitor banks are switched in and out of circuits as the demand for capacitive VAR compensation of the load fluctuates. There are also other devices, such as filters, etc., in industrial distribution systems that include power capacitors.
A resonance oscillation can occur in any circuit containing inductors and capacitors. For the simplest L-C circuit, shown in FIG. 1, closing the switch will cause the circuit to oscillate at its natural resonance frequency (f) determined by: ##EQU1##
A practical power circuit usually includes multiple inductive and capacitive devices, and therefore the circuit can be resonant at multiple frequencies. A resonant oscillation in a practical circuit caused by a switching operation may not be sustained for a long period due to the losses in the circuit. However, the oscillation in the circuit could be sustained if harmonic sources are present in the circuit and one of the circuit resonant frequencies becomes the same as one of the harmonic source frequencies. Such a sustained harmonic resonance may cause significant harmonic distortion in the system voltages and currents, which may increase the losses in the circuit and cause damage to equipment operating in the system due to overheating and vibration.
Capacitor switching changes the topology of a circuit, which may tune the circuit to a resonance condition. This may result in sustained resonance oscillations in the circuit if one of the circuit frequencies is tuned to one of the harmonic source frequencies. The harmonic resonance could occur when a capacitor bank is switched into the circuit or when it is switched out of the circuit. FIGS. 2, 3 and 4 depicts waveforms exhibiting such resonance conditions. In FIGS. 2 and 3, energizing the capacitor results in a harmonic resonance, while in FIG. 4 switching off (de-energizing) the capacitor causes harmonic resonance to occur.
The waveforms depicted in FIGS. 2-4 were obtained from a simulated industrial distribution system of the kind shown in FIG. 5. The system of FIG. 5 includes a plurality of buses (numbered 1 through 9), loads and capacitors (denoted SC1, SC2 and SC3). The waveforms of FIGS. 2-4 were obtained when capacitor SC3 at bus 9 was switched on and off. It should be noted that the resonance oscillation could also be caused by other changes in the system, such as switching a circuit in or out of service, etc.
The most common solution to the resonance problem in the past has been to carefully design the parameters of the capacitor(s) to ensure that the circuit resonance frequencies stay away from the harmonic source frequencies. This usually requires a system study to be conducted for each power capacitor application. Other examples of prior art are cited in the Information Disclosure Statement filed herewith.